Accelerator-based radiation therapy typically generates a high energy X-ray beam via bremsstrahlung (“braking radiation”). A relativistic electron beam is incident on a target material of high atomic number (“high Z”). The electrons are deflected (accelerated) by electromagnetic interactions with the target nuclei, causing emission of high energy photons. Some of these photons have enough energy to create electron-positron pairs, which then interact with target nuclei to emit more photons. The result is an “electromagnetic shower” or “electromagnetic cascade” of electrons, positrons, and photons. Any electrons which escape the target are typically eliminated from the therapy beam by an electron absorber made of low Z material (e.g. aluminum, carbon).